A positive temperature coefficient (PTC) element exhibits a PTC effect that renders the same to be useful as a circuit protection device, such as a resettable fuse. The PTC circuit protection device includes a PTC element and first and second electrodes attached to two opposite surfaces of the PTC element. The PTC element includes a polymer matrix that contains a crystalline region and a non-crystalline region, and a particulate conductive filler, e.g., carbon-based filler, dispersed in the non-crystalline region of the polymer matrix and formed into a continuous conductive path for electrical conduction between the first and second electrodes. The PTC effect is a phenomenon where crystals in the crystalline region start to melt when the temperature of the polymer matrix is raised to its melting point, resulting in the generation of a new non-crystalline region. As the new non-crystalline region is increased to an extent to merge into the original non-crystalline region, the conductive path of the particulate conductive filler will become discontinuous and the resistance of the PTC element will be sharply increased, thereby resulting in an electrical disconnection between the first and second electrodes.
The PTC circuit protection device is required to exhibit a high positive temperature coefficient effect, high conductivity, and high electrical stability.
The polymer matrix of the conventional PTC element is made from a polymer composition containing non-grafted high density polyethylene (HDPE) and, optionally, carboxylic acid anhydride-grafted HDPE. The non-grafted HDPE has a weight average molecular weight ranging from 50,000 g/mole to 300,000 g/mole and a melt flow rate ranging from 0.01 g/10 min to 10 g/10 min according to ASTM D-1238 under 190° C. and a load of 2.16 Kg. The carboxylic acid anhydride-grafted HDPE has a weight average molecular weight ranging from 50,000 g/mole to 200,000 g/mole and a melt flow rate ranging from 0.5 g/10 min to 10 g/10 min according to ASTM D-1238 under 190° C. and a load of 2.16 Kg. The carboxylic acid anhydride-grafted HDPE functions to increase adhesion between the PTC element and the electrodes.
The carbon-based filler is not suitable for a PTC circuit protection device which requires high conductivity and relatively high maximum endurable voltage. The maximum endurable voltage may be improved by the addition of a non-conductive filler, e.g., an inorganic material. However, the conductivity of the PTC circuit protection device is still unsatisfactory.